1. Field of the Invention
The present invention relates to an electrophotographic photoconductor used for electrophotographic type image formation and to an image forming apparatus provided therewith.
Particularly, the present invention relates to an electrophotographic photoconductor used for electrophotographic type image formation and particularly, for full-color image formation or multicolor image formation, for example, a color printer and to an image forming apparatus provided with the electrophotographic photoconductor.
More particularly, the present invention relates to a tandem system image forming apparatus provided with plural photographic photoconductors and plural developing units each receiving a developer having a different color.
2. Description of Related Art
Electrophotographic type image forming apparatuses such as copying machines, printers and facsimile machines are generally provided with a “electrophotographic photoconductor” (hereinafter, also referred to simply as “photoconductor”), a charging means, an exposure means, an electrostatic latent image formation means, a developing means, a transfer means and a fixing means and further, a means for recovering a toner left unremoved on the surface of the photoconductor after the transfer action of the transfer means.
Usually, the above photoconductor has a structure in a photosensitive layer containing a photoconductive material on a conductive substrate made of a conductive material.
Also, examples of the above photoconductor include inorganic photoconductors such as a selenium-based photoconductor using, for example, amorphous selenium (a-Se) or amorphous arsenic selenium (a-AsSe) as the photosensitive layer; a zinc oxide-based photoconductor using zinc oxide (ZnO) or cadmium sulfide-based photoconductor using cadmium sulfide (CdS) as the photosensitive layer; and an amorphous silicon-based photoconductor (a-Si) using amorphous silicon (a-Si photoconductor) as the photosensitive layer, and photoconductors (hereinafter also referred to as organic photoconductor) using organic photoconductive materials, that is, organic photoconductors (abbreviation: OPC).
These organic photoconductors have many advantages in the points of, for example, toxicity, manufacturing cost and freedom of material designing though they have some non-negligible problems concerning sensitivity, durability and stability to environments.
Moreover, the organic photoconductors have the characteristics that they can be formed using an easy and inexpensive method represented by, for example, the dip coating method and therefore, stand the mainstream in the field of photoconductors at present.
As the structure of such an organic photoconductor, a variety of structures are proposed which include a monolayer structure formed by dispersing both a charge generation material and a charge transport material (also referred to as “charge transfer layer”) in a binding resin (also referred to as “binder resin”) on the conductive substrate made of conductive materials and a laminate structure in which a charge generation layer obtained by dispersing a charge generation material in a binding resin and a charge transport layer obtained by dispersing a charge transport material in a binding resin on the conductive substrate in this order or a reverse two-layer type laminate structure obtained by laminating these layers in inverse order.
Particularly, the performance of the organic photoconductor has been significantly improved with development of a separated function type photoconductor in which a charge generation function and a charge transport function are assigned to separate materials.
Specifically, the separated function type photoconductor has, besides the above advantages, the advantages that the materials constituting the photosensitive layer can be selected from a wide range of materials, so that a photoconductor having desired characteristics can be manufactured relatively easily.
The function separation type photoconductors include a laminate type and a monolayer type.
The laminate function separation type photoconductor is provided with a laminate type photosensitive layer constituted by laminating a charge generation layer containing a charge generation material carrying a charge generation function and a charge transport layer containing a charge transport material carrying a charge transport function.
The charge generation layer and the charge transport layer are usually formed by dispersing a charge generation material and a charge transport material each in a binder resin which is a binder.
The monolayer function separation type photoconductor is, on the other hand, provided with a monolayer type photosensitive layer obtained by dispersing a charge generation material and charge transport material together with a binding resin.
Also, in an electrophotographic device, the above charging, exposing, developing, transfer, cleaning and static elimination operations are repeated in various environments.
It is therefore demanded of the photoconductor to have high sensitivity and high light responsibility and besides, to be superior in environmental stability, electric stability and durability to mechanically external force (scratch resistance).
Specifically, the photoconductor needs to have high scratch resistance which is a resistance to the abrasion of the surface layer which is caused by the sliding friction of a cleaning member and the like.
In the meantime, when a full-color image or a multicolor image is formed by using an electrophotographic process, plural developing units each having a different color toner are provided in the apparatus and the formed toner images are made to be overlapped on each other to thereby form a color image or a multicolor image. In recent years, a tandem system image forming apparatus in which plural photoconductors corresponding to each toner color are arranged in a line has come to be used to raise printing speed.
In the tandem system image forming apparatus, plural component color images corresponding to a full-color image or multicolor image information are each formed on each photoconductor and laminated sequentially on recording paper, and transferred to the recording paper to output an image formation product in which the full-color image or multicolor image is synthesized and reproduced. Such a tandem system image forming apparatus is useful as a full-color forming apparatus, a multicolor image forming apparatus or an image forming apparatus capable of forming both full-color and multicolor images.
The tandem system image forming apparatus capable of forming a full-color or multicolor image has a structure which generally includes four developing units for receiving cyan, magenta, yellow and black colors respectively, and four photoconductors consisting of a cyan photoconductor used to form a cyan-color image, a magenta photoconductor used to form a magenta-color image, a yellow photoconductor used to form a yellow-color image and a black photoconductor used to form a black-color image. In an image forming apparatus, not only full-color images or multicolor images but also monochromatic images such as white and black images are frequently formed. Therefore, the practical control of operation is changed according to the image formation mode designated by users to limit the abrasion of a photoconductor.
For example, when the color image output mode is designated by a user, the position of each photoconductor for cyan, magenta, yellow and black colors is adjusted to the working position in contact with the transfer and handler belt and each photoconductor is driven with rotation to perform a charge action, an exposure action and a developing action for the photoconductor, thereby transferring a toner image formed on the photoconductor to recording paper. A full-color image is thus formed on the recording paper by these operations.
Also, when a white and black image output mode is designated, first, a releasing mechanism is driven to make each photoconductor for cyan, magenta and yellow colors apart from the transfer conveyer belt and then made to stop its rotation. Furthermore, the charge action, exposure action and developing action for each photoconductor are stopped. In this state, only the black photoconductor is driven with rotation to perform a charge action, an exposure action and a developing action for the black photoconductor to transfer the black toner image formed on the black photoconductor to the recording paper. A white and black image is thus formed on the recording paper by these operations.
As mentioned above, when a white and black image output mode is designated, photoconductors other than a black photoconductor is made to be apart from the transfer conveyer belt and is made to stop its rotation, whereby the abrasive actions of a cleaning blade, recording paper, transfer conveyer belt and the like on photoconductors unnecessary when the white and black image output mode is designated can be limited.
This action control is more effective when the frequency of formation of a full-color image is higher than that of formation of a white and black image. However, in an image forming apparatus used actually, the frequency of formation of a white and black image is higher than that of formation of a full-color image, and therefore, the rate of abrasion is relatively higher in the case of the black photoconductor than in the case of other color photoconductors. In usual, the characteristics of a photoconductor largely depend on a reduction in film thickness and are more deteriorated along with a reduction in film thickness. Therefore, when the rate of abrasion of the black photoconductor is higher than that of other color photoconductors, the black photoconductor is deteriorated at a higher rate than other color photoconductors. Therefore, there is the problem that a full-color image is unevenly colored with an increase in the number of copies.
The problem of this color unevenness can be solved by exchanging the deteriorated black photoconductor for a new one. However, when only the deteriorated black photoconductor is replaced with a new one, a color balance between a toner image formed using the exchanged black photoconductor and a toner image formed using other color photoconductors is upset, so that no full-color image having high quality can be obtained.
Therefore, when the black photoconductor is deteriorated, it is necessary to replace all other non-deteriorated color photoconductors with new ones. In other words, each workable time of four photoconductors to be used for formation of each color image is controlled by the available time of the black photoconductor having the highest frequency of use, which is a large waste of time and money, leading to high running costs.
In order to limit an increase in running costs, it is necessary to prolong the life of the black photoconductor which determines the life of the device so that the black photoconductor can be used for the same period of time as other color photoconductors. For this, for example, such technology have been proposed that the film thickness of the photosensitive layer of the black photoconductor is made to be higher than that of each photosensitive layer of other color photoconductors (see Publication of JP-A 2000-242056) and that only the black photoconductor is designed to have a larger diameter (see Publication of JP-A 2000-242057).
When, similarly to the image forming apparatuses of Patent Documents 1 and 2, only the black photoconductor is designed to have a different structure to prolong its life so as to be able to use the black photoconductor for the same period of time as other color photoconductors, it is necessary that the means such as the charging means, exposure means, and transfer means used for the black photoconductor have structures different from those used to form other color images similarly to the case of using an α-Si photoconductor or an amorphous silicon carbon (α-SiC)-based photoconductor, showing that these technologies of Documents 1 and 2 have poor productivity and are increased in production costs.
Particularly, if the film thickness of the photosensitive layer or charge transport layer of the black photoconductor is increased as described in the technology of Publication of JP-A 2000-242056, this gives rise to problems that the black photoconductor is deteriorated in chargeability and resolving power. Also, if the diameter of the black photoconductor is increased as described in Publication of JP-A 2000-242057, this gives rise to a problem that the size of the apparatus body is large.
In order to prolong the available time of the four photoconductors used so as to form each color image, not only the black photoconductor is designed to prolong its life and to use the black photoconductor for the same period of time as other color photoconductors but also it is necessary to prolong each life of all photoconductors including the black photoconductor. Therefore, all photoconductors including the black photoconductor need to have high mechanical durability and be resistant to abrasion, that is, to have the surface layer having high mechanical durability.
With regard to the technology for improving the mechanical durability of the surface layer of the photoconductor, there is, for example, such a proposal that a copolymer polycarbonate resin is compounded in the surface layer of a photoconductor (see Publication of JP-A 2000-330303).
In the meantime, in a general electrophotographic type image forming apparatus, many photoconductors are not a little affected adversely by ozone generated from the apparatus and NOx generated from, for example, a charging device.
As regard to the influences of ozone and NOx, these harmful materials cause particularly under the circumstance of high temperature and high relative humidity, a reduction in charged potential, a rise in residual potential and a reduction in surface resistance, a deterioration in resolution and a significant deterioration in the qualities of an output image, with the result that there is the case where the life of the photoconductor is shortened.
Also, when the image forming apparatus is allowed to stand for a long period of time under a low-humidity environment, there is the case where color unevenness is caused when the apparatus is started after it is allowed to stand. This poses the problem that ozone and NOx are left collectively only just under the charging device of the photoconductor when the image forming apparatus is stopped and therefore, the surface potential of this part is dropped, resulting in lowered image density.
Also, in the case of a photoconductor having low mechanical durability, that is, a photoconductor having an easily peelable surface layer, the film is peeled off before the damages caused by ozone and NOx are accumulated and therefore, the above problems are scarcely tangible. However, if the mechanical durability is heightened, the surface layer is not abraded that much and therefore, the damages caused by ozone and NOx are easily accumulated and become more conspicuous.
Also, the fluidity of gases around the photoconductor is impaired in current apparatuses designed for space saving and therefore, ozone and NOx are easily collected, which is the reason why it is desired to solve these problems.
Also, the aforementioned problem that color unevenness arises when the apparatus is started after it is allowed to stand under a low-humidity environment is serious particularly in a tandem system color machine.
Specifically, in the tandem system, plural component color images corresponding to a full-color image or multicolor image information are each formed on each photoconductor and laminated sequentially on recording paper, and transferred to the recording paper to output an image formation product in which the full-color image or multicolor image is synthesized or reproduced. Therefore, when a part decreased in density arises partly in each photoconductor, this brings about a difference in the position where color unevenness occurs in every color, causing the possibility of formation of a very unbalanced image, which is fatal to a color image forming apparatus.
In order to solve these problems, it is effective to add additives such as an antioxidant. It is considered to be effective to increase the amount of these additives in particular the tandem system color machine for higher effects of these additives.
However, if the amount of the additive is increased, this brings about a reduction in density in long-term use because of a rise in residual potential in repeat use.
In light of this, it is desired to use an additive having an excellent effect even in a small amount to prevent a reduction in density in long-term use.
The above Publication of JP-A 2000-330303, also from this point of view, is not a measures taken to solve these problems even if it contributes to an improvement in mechanical durability.
In the use of an additive in a color image forming apparatus, it is proposed to use a phenol-based or thioether-based antioxidant containing a sulfur atom in a combination of a specified charge generation material and a charge transport material (see Publication of JP-A 2004-12718). However, these methods are also unsatisfactory and a tandem system color image forming apparatus is desired to be improved in mechanical durability and to provide a stable image quality for a long period by using additives having excellent effect even if the amount of these additives is small.
Also, the durability of an electrophotographic photoconductor can be improved by using a polyarylate resin. However, resins deteriorated by oxidation and substances stuck to the surface are insufficiently removed, which is a cause of reduction in surface resistance of the photoconductor, giving rise to the so-called image deletion which is a phenomenon that an image is blurred as if it is deleted a flowing image particularly under a high-temperature and high-humidity condition in the same manner as in the case where a protective layer is contained or a filler is contained in the surface of the photoconductor.
A method in which an antioxidant is added in the charge transport layer is adopted as the measures taken to prevent this image deletion. However, there is a problem that the addition of an excess amount of the antioxidant causes a reduction in sensitivity by repeated operations. In order to improve these problems, the temperatures of a photoconductor support and a charge transport layer coating solution are controlled to thereby increase the adhesion between the charge generation layer and the charge transport layer to thereby improve charge injection efficiency (Publication of JP-A 2004-317944). However, this improvement is not said to be sufficient and a more excellent antioxidant is desired.
Specifically, the above-mentioned conventional art have failed to provide a photoconductor having sufficient effect of oxidation-resistant gases and a color tandem image forming apparatus. Also, the addition of such an antioxidant allows a harmful effect to still remain, so that it provides practically unsatisfactory characteristics, bringing about deteriorations in the electrophotographic characteristics such as a reduction in sensitivity and a rise in residual potential.
Therefore, a proposal of a novel material which is improved in resistance to oxidizing gas and is free from harmful effects on the electrophotographic characteristics and an image forming apparatus using that material are expected.